1. Filed of the Invention
The present invention relates to a conductivity modulation type MOSFET used as a power switching device.
2. Description of the Related Art
The market in power switching devices continues to demand performance characterized by high speed, high breakdown voltage, and high power. As a result, large power MOSFETs (insulated, large power MOSFETs) have found application as power switching devices and are rapidly replacing older components. A conductivity modulation type MOSFET (insulated gate bipolar transistor (IGBT)) is a switching device superior to a conventional bipolar transistor in breakdown voltage, power, and operating speed. The IGBT is used particularly in the field of invertor control, but its application is expanding to other fields.
A basic structure of an n-channel IGBT is shown in FIG. 2. The n-channel IGBT may be considered a high power MOSFET, a so called vertical DMOS, in which an n.sup.+ region serving as a drain region is replaced by a p.sup.+ collector region 1. Multiple p-base regions 3 are selectively formed in the surface region of an n.sup.- region 2 in contact with the p.sup.+ region 1. Two n.sup.+ source regions 4 are formed in the surface of each p-phase region 3. Additionally, a p.sup.+ well region 5, which extends deeper than the p region 3, is formed in the central portion of the p-base region 3. In order to form an n-channel in a p-base region 30 located between the exposed portions of the n.sup.+ source region 4 and the n.sup.- drift region 2, a gate electrode 7 is provided, which is connected through an insulating film 6 to a gate terminal G. An emitter electrode 8, which is connected to an emitter terminal E through a contact hole opened in the insulating film 6, is in contact with the p.sup.+ well 5 and the n.sup.+ source region 4. A collector electrode 9, which is connected to a collector terminal C, is connected to the p.sup.+ region 1. When the emitter terminal E of the IGBT is grounded, and a positive voltage potential is applied to the gate terminal G and the collector terminal C, the surface of the p-base layer 3 located just under the gate electrode 7 is inverted to form a channel of electrons on the same principle as that of the high power MOSFET. Under this condition, the n.sup.- region 2 is equivalently grounded and hole current is injected form the p.sup.+ collector region 1 into the n.sup.- region. In other words, minority carriers (holes) are injected into the high resistance layer n.sup.- region. The injection of the minority carriers increases the concentration of electrons so as to satisfy the charge neutrality condition, and significantly reduces resistance of the n.sup.- region. Thus, the IGBT serves as a power switching device of satisfactorily low ON-resistance because of the conductivity modulation effect.
Several problems are apparent in the present use of the IGBT as a power switching device.
As shown in FIG. 3, an emitter current I.sub.E is expressed by EQU I.sub.E =I.sub.h +I.sub.MOS ( 1)
Assuming that the current gain of a stray pnp bipolar transistor 21 consisting of the p-base region 3, n.sup.- drift region 2, and p.sup.+ collector region 1 is .alpha..sub.PNP, the hole current I.sub.h is EQU I.sub.h ={.alpha..sub.PNP /(1-.alpha..sub.NPN)}I.sub.MOS ( 2)
where I.sub.MOS is electron current. Substituting equation (2) into equation (1) and rearranging results in EQU I.sub.E ={1/(1-.alpha..sub.PNP)}I.sub.MOS ( 3)
As seen from equation (2), the hole current I.sub.h, viz., the current of the IGBT, changes depending on the value of .alpha..sub.PNP.
FIG. 4 is a graph showing a typical switching waveform of the bipolar transistor at the time it is turned OFF. As shown, the switching operation progresses through two phases. During a first phase 41, the channel disappears and electron current becomes zero. Accordingly, the current reduces by this amount. During a second phase 42, the current caused by the pn.sup.- p bipolar transistor diminishes because of the recombination of carriers left in the n.sup.- layer due to the carrier lifetime .tau. in an open base state. Accordingly, the second phase is determined by an injection level of the hole current or the carrier lifetime .tau.;
To design the bipolar transistor operating at high frequency, a designer employs any of the following approaches: 1) The injection level of hole current is controlled. To this end, a buffer layer n.sup.+ layer is formed between the p.sup.+ substrate and the buffer n.sup.- high resistance region. (for example, see IEEE, IEDM Technical Digest, 4.3 (1983) pp. 79 to 82). 2) A concentration of the p.sup.+ substrate is controlled. 3) The carrier lifetime .tau. is reduced by using a lifetime control process, such as electron beam irradiation and heavy metal diffusion (For example, see IEEE, Trans. Electron Devices, ED-31 (1984) pp. 1790 to 1795). Each of these conventional approaches requires some trade-off with the ON-voltage of the bipolar transistor. The trade-off could be significantly reduced if a process existed to pull out the carrier to the p.sup.+ substrate region or to another electrode.
The IGBT has another problem in addition to a stray, pnp bipolar transistor 21. As shown in FIG. 3, another stray, npn bipolar transistor 22 exists. This npn bipolar transistor consists of an n.sup.+ source region 4, p-base region 3, and n.sup.- drift region 2. These stray transistors 21 and 22, have current gains .alpha..sub.PNP and .alpha..sub.NPN, respectively, and cooperate to substantially form an npnp thyristor structure.
Accordingly, a so-called latching phenomenon can occur in which the thyristor turns on when the sum of the current gains is greater than or equal to 1, viz., .alpha..sub.PNP +.alpha..sub.PNP .gtoreq.1. Once latching occurs, the IGBT loses gate control over the current and in extreme cases will be destroyed. These extreme cases of latching abruptly destroying the IGBT must be eliminated, particularly in the application of invertor control.
Typical measures thus far taken against the latching phenomenon are: 1) reducing base resistance in the p.sup.+ well 5 to check activation of the stray transistors (For example, see IEEE Trans. Electron Devices, ED-32 (1985), p2554), 2) reducing the majority carrier in the p-base layer 3, and 3) reducing current concentrated in the region near the emitter/base junction of the element (For example, see U.S. Pat. No. 4,809,045). Nevertheless, a destruction level (load short-circuiting mode) of an IGBT incorporating any of the above measure has not yet reached that of the conventional bipolar transistor. However, the risk of the latching phenomenon could be reduced if the current gain of either stray bipolar transistor 21 or 22 is markedly reduced.